C++ | References | Question 1

What is the return value of f(p, p) if the value of p is initialized to 5 before the call? Note that the first parameter is passed by reference, whereas the second parameter is passed by value.

int f(int &x, int c) {
   c  = c - 1;
   if (c == 0) return 1;
   x = x + 1;
   return f(x, c) * x;
} 

(A) 3024
(B) 6561
(C) 55440
(D) 161051


Answer: (B)

Explanation: Since c is passed by value and x is passed by reference, all functions will have same copy of x, but different copies of c.

f(5, 5) = f(x, 4)*x = f(x, 3)*x*x = f(x, 2)*x*x*x = f(x, 1)*x*x*x*x = 1*x*x*x*x = x^4

Since x is incremented in every function call, it becomes 9 after f(x, 2) call. So the value of expression x^4 becomes 9^4 which is 6561.

#include <stdio.h>

int f(int &x, int c)
{
    c  = c - 1;
    if (c == 0) return 1;
    x = x + 1;
    return f(x, c) * x;
}
int main()
{
    int p = 5;
    printf("%d", f(p, p));
}

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